The invention relates to temperature sensors or temperature sensor arrangements made from glass ceramic and bonding film resistors, which are suitable especially for control and limitation of output in glass ceramic cooking areas.
The use of glass ceramics as temperature sensors is already known from DE-PS 21 39 828. In this publication, especially a temperature-measuring resistor made from a glass ceramic of the SiO.sub.2 -Al.sub.2 O.sub.3 -Li.sub.2 O system is described. This glass ceramic is distinguished by a high resistance to thermal shocks because its very low thermal expansion, .alpha., is less than or equal to 1.5 .multidot. 10.sup.-6 /K, and is therefore suitable especially for use as a measuring probe in very hot industrial furnaces, waste gas chambers, etc.
To monitor temperatures of large surfaces, a sensor is described in the publication which is distinguished in that strip-like glass ceramic areas are delimited and bonded on glass ceramic supports made flat with strip conductors made from Au, Pt or Ag, whose electrical resistance is used as a temperature-measuring resistor According to the publication, the total resistance of this arrangement can be considered to consist of a multiplicity of differential resistor elements connected in parallel, and the least resistance occurs where the surface is heated the most. The low heat conduction of the glass ceramic prevents a quick equalization of temperature of the overheated point with the environment. The total resistance of this arrangement is determined by the resistance of the hottest point of the surface, until the temperature equalization has taken place. Local overheating thus produces short-term, extreme resistance changes of the temperature-measuring resistor, which can be used according to the publication, for example, to control an overheating safety device.
U.S. Pat. No. 4,237,368 describes a use of the above-described sensors for monitoring the temperature of cooking zones in glass ceramic cooking surfaces with the help of electric comparators. For this purpose, strip-like glass ceramic areas, which run along a diameter of the cooking zones, are delimited and bonded as temperature sensors in the cooking zones with two parallel strip conductors each made from gold. The bonded strip conductors are bridged with a shunt resistor connected in parallel on their outermost ends, which achieve a thermoelement safeguarding against breakage in cold cooking zones. One each of the two strip conductors of the temperature sensors is connected to a strip conductor, which goes around the entire cooking surface along its periphery and acts as a broken glass sensor.
DE-OS 37 44 373 A 1 describes an electronic power control arrangement and process for a glass ceramic cooking area, which uses sensors to determine temperatures according to the above-described arrangement of U.S. Pat. No. 4,237,368. The heat output given off respectively in the cooking zones is controlled in this case as a function of the glass ceramic temperature and its rate of change. For this purpose, the strip-like glass ceramic resistances delimited in the cooking zones by parallel gold strip conductors deliver the necessary temperature signals because of their change of resistance with the temperature.
An improved output control and monitoring arrangement for glass ceramic cooking areas, which in principle corresponds to the arrangement of the above-described type relative to the temperature sensor arrangements in the cooking zones, is also known from DE-OS 37 44 372.
The specific resistance of all known glass ceramics, which are used as cooking surfaces, at 20.degree. C. is on the order of magnitude of 10.sup.13 .OMEGA..multidot. cm to 10.sup.15 .multidot..OMEGA. cm and reaches values between 10.sup.7 .OMEGA..multidot. cm to 10.sup.9 .OMEGA..multidot. cm. at temperatures around 200.degree. C. Independently of the respective sensor geometry, the temperature-measuring signal--or sensor current or the voltage drop on a measuring resistor with the sensor connected in series--thus is changed in said temperature range by 6 powers of ten. For evaluation and further processing of these temperature-measuring signals, expensive electronic precision instruments, which are equipped with measuring range switches and/or logarithmic amplifiers, are necessary, which cannot be used in kitchen ranges. This is aggravated by the fact that the large-area and very high-ohmic sensors, which are placed in direct proximity with the heating coils or cooking zone heating, are sensitive to capacitively or inductively interspersed spurious signals for whose removal an additional electronic expenditure has to be made.
Because of the described problems, all glass ceramic temperature sensors, described in the "prior art," for control and adjustment of the cooking zone temperature of glass ceramic cooking surfaces are not suitable in the temperature range below 200.degree. C. This is a considerable drawback, since to keep hot, to melt fats and in many other uses of glass ceramic cooking areas, the exact observance of cooking zone temperatures below 200.degree. C. is necessary and of great importance for the user.
Another drawback of the sensors known from the above-mentioned publications is that they are placed respectively only along a diameter or half diameter in the cooking zones and because of the already mentioned low heat conduction of the glass ceramic, it is on the order of magnitude of 2 to 3 W/mK; (watts/milliKelvin only the temperatures in these linear partial areas of the cooking zones can be detected. Thus, neither is a sufficient overheating protection of the total cooking zone assured nor is the temperature signal, supplied by the linear partial areas, relative to the actual heat flow to the entire pot bottom assured, so that the heating devices with the sensors of the above-mentioned publications are controlled by a temperature signal, which does not correspond to the temporary removal of heat from the entire cooking zone.
Further protective temperature limitation devices are known (e.g., according to DE-PS 37 05 260.8 or DE-PS 37 05 261.6), which are placed as separate component at a short distance below the cooking zones from the glass ceramic cooking surfaces along a diameter of the cooking zones. They are heated both by the radiant heating element contained below them and by the radiation from the underside of the cooking surfaces. They consist of a metal rod, with great thermal expansion, which is placed in a pipe made from a material with low thermal expansion. A spring-controlled switch is actuated when a specified temperature is reached because of the relative thermal expansion between the metal rod and the pipe. This leads to energy timing of the heating element within a narrow temperature range, which is determined by the switching hysteresis of the spring-controlled switch.
The temperature limitation device responds to the integral temperature of the heating element/cooking surface system. The radiant heating on the side of the cooking surface imparts the input to the superposed pot.
A drawback of this limitation device arrangement is that, here also, the heating on the cooking surface side of the limiting rod primarily is the result of the radiation of the linear diameter area of the cooking zone placed directly over it. If much heat is removed by a superposed pot from this diameter zone, for example, in the edge area, the limitation device remains colder there than in its central area. The limiting rod thus undergoes an apparent shortening, since for the switching, now basically only the hotter central area is active. Thus, in the case of constant switching characteristics, the switching point, consequently also the limiting temperature, is raised. By uneven radiant heating on the side of the cooking surface, the level of the limiting temperature thus is influenced in proportion to the respective "active limiting rod length" coming into effect. In unfavorable cases, this can result in overheating of the cooking zones.